netdev - Re: [RFC PATCH] fix IP_ECN_set

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Date:	Wed, 19 Dec 2012 16:41:24 +0800
From:	RongQing Li <roy.qing.li@...il.com>
To:	Julian Anastasov <ja@....bg>
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC PATCH] fix IP_ECN_set_ce

2012/12/19 Julian Anastasov <ja@....bg>:
>
>         Hello,
>
> On Wed, 19 Dec 2012, roy.qing.li@...il.com wrote:
>
>> From: Li RongQing <roy.qing.li@...il.com>
>>
>> 1. ECN uses the two least significant (right-most) bits of the DiffServ
>> field in the IPv4, so it should be in iph->tos, not in (iph->tos+1)
>>
>> 2. When setting CE, we should check if ECN Capable Transport supports,
>> both 10 and 01 mean ECN Capable Transport, so only check 10 is not enough
>>     00: Non ECN-Capable Transport — Non-ECT
>>     10: ECN Capable Transport — ECT(0)
>>     01: ECN Capable Transport — ECT(1)
>>     11: Congestion Encountered — CE
>>
>> 3. Remove the misunderstand comment
>>
>> 4. fix the checksum computation
>>
>> Signed-off-by: Li RongQing <roy.qing.li@...il.com>
>> ---
>>  include/net/inet_ecn.h |   22 ++++------------------
>>  1 file changed, 4 insertions(+), 18 deletions(-)
>>
>> diff --git a/include/net/inet_ecn.h b/include/net/inet_ecn.h
>> index aab7375..545a683 100644
>> --- a/include/net/inet_ecn.h
>> +++ b/include/net/inet_ecn.h
>> @@ -73,27 +73,13 @@ static inline void INET_ECN_dontxmit(struct sock *sk)
>>
>>  static inline int IP_ECN_set_ce(struct iphdr *iph)
>>  {
>> -     u32 check = (__force u32)iph->check;
>> -     u32 ecn = (iph->tos + 1) & INET_ECN_MASK;
>> -
>> -     /*
>> -      * After the last operation we have (in binary):
>> -      * INET_ECN_NOT_ECT => 01
>> -      * INET_ECN_ECT_1   => 10
>> -      * INET_ECN_ECT_0   => 11
>> -      * INET_ECN_CE      => 00
>> -      */
>
>         I think, the above comment explains how an
> increment (iph->tos + 1) serves the purpose to check
> for ECT_1 and ECT_0, there is no such thing as
> addressing the next byte from header. It is just an
> optimized logic that avoids complex INET_ECN_is_XXX
> checks.
Thanks for your reply.
Do you mean this comment are valuable?


>
>> -     if (!(ecn & 2))
>> +     u32 ecn = iph->tos & INET_ECN_MASK;
>> +
>> +     if (INET_ECN_is_ce(ecn) || INET_ECN_is_not_ect(ecn))
>>               return !ecn;
>
>         May be return INET_ECN_is_ce(ecn) ?
>

I like to set the return value to void, since noone cares about the
return value.

-Roy

>>
>> -     /*
>> -      * The following gives us:
>> -      * INET_ECN_ECT_1 => check += htons(0xFFFD)
>> -      * INET_ECN_ECT_0 => check += htons(0xFFFE)
>> -      */
>> -     check += (__force u16)htons(0xFFFB) + (__force u16)htons(ecn);
>> +     csum_replace2(&iph->check, iph->tos, iph->tos|INET_ECN_CE);
>>
>> -     iph->check = (__force __sum16)(check + (check>=0xFFFF));
>>       iph->tos |= INET_ECN_CE;
>>       return 1;
>>  }
>> --
>> 1.7.10.4
>
> Regards
>
> --
> Julian Anastasov <ja@....bg>
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